1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing part for fixing a toner image to a recording material.
2. Description of the Related Art
Conventionally, for an image forming apparatus, such as a copier or a laser beam printer, the following fixing apparatus has been used as a fixing apparatus for heating a toner image formed on a recording material and fixing the toner image thereto. For example, a heat-fixing apparatus of a heat-roller type which uses a halogen lamp as a heat source or a heat-fixing apparatus of a film heating type which uses a ceramic heater as a heat source is used.
In general, a heater is connected to an AC power supply via a switching element such as a triac, and is supplied with power by the AC power supply. The fixing apparatus is provided with a temperature detection element, for example, a thermistor temperature sensing element. The temperature of the fixing apparatus is detected by the temperature detection element. Then, based on detected temperature information, a central processing unit (CPU) performs on/off control on the switching element, to thereby turn on/off power supplied to the heater, which enables such temperature control that sets the temperature of the fixing apparatus to a target temperature. The on/off control of the heater is performed by one of phase control and wave number control.
The phase control method is a method of supplying power to the heater by turning on the heater at an arbitrary phase angle within one half-wave of an AC wave form. Meanwhile, the wave number control method is a power control method in which the heater is turned on/off in units of half-wave of the AC wave form. Most of conventional technologies use one of the phase control and the wave number control.
The reason for selecting phase control is possibly because flickering of a lighting apparatus, which is the phenomenon called flicker, may be suppressed. Flicker refers to the flickering of the lighting apparatus when the AC power supply generates voltage fluctuations due to fluctuations in a load current of an electrical apparatus connected to the same power supply as the lighting apparatus and an impedance of a distribution line. Phase control is such control that the switching element is turned on midway through one half-wave (phase angle ranging from 0° to 180°). Therefore, the change amount and the change period of the current are small, which may suppress the occurrence of the flicker. Meanwhile, wave number control is such control that the switching element is turned on at a zero-crossing point of the AC wave form. Therefore, the fluctuations in the current are larger than in phase control, and hence flicker is more likely to occur.
The reason for selecting wave number control is possibly because a harmonic current and switching noise may be suppressed. The harmonic current and switching noise are generated due to steep fluctuations in current caused when the heater is turned on/off. This is because the harmonic current and switching noise are generated to a smaller extent in wave number control in which the on/off control of the heater is always performed at the zero-crossing point than in the phase control in which switching is performed midway through the half-wave of the AC wave form. The harmonic current and switching noise tend to be generated to a larger extent with a higher voltage of the AC power supply being used.
It is therefore general to set a control method depending upon an AC commercial power supply voltage in a region in which the image forming apparatus is used. For example, the control of the heater is performed by choosing the phase control method effective for flicker for the region using an AC commercial power supply voltage of, for example, 100 V to 120 V. Meanwhile, the control of the heater is performed by choosing the wave number control method effective for the harmonic current and the switching noise for the region using an AC commercial power supply voltage of, for example, 220 V to 240 V. In such a manner, the control of the heater is generally fixed to one of the methods.
Further, there is a technology that proposes a method combining the phase control and the wave number control. For example, in Japanese Patent Application Laid-Open No. 2003-123941, a plurality of half-waves are set as one control period, partial half-waves of the one control period being subjected to the phase control and the remaining half-waves being subjected to the wave number control. This may prevent the generation of the harmonic current and the switching noise to a smaller extent than in the case of using only the phase control. In addition, flicker may be reduced to a lower level than in the case of using only wave number control, which allows multistage control of the power to the heater.
Here, a positive half-wave at which the power is supplied by one of the phase control and the wave number control is defined as a positive energization cycle, while a negative half-wave at which the power is supplied thereby is defined as a negative energization cycle. Further, a half-wave at which the power is not supplied is defined as a non-energization cycle. Further, one unit period for controlling the amount of power to be supplied to the heater by separating the amount by a fixed period is defined as one control period.
When controlling the temperature of the fixing apparatus, a sequence controller compares the temperature detected by the temperature detection element with the preset target temperature, and calculates a power duty (power ratio) of the above-mentioned heater. Then, the sequence controller determines one of the phase angle and the wave number corresponding to the power duty, and, under one of a phase condition and a wave number condition thereof, controls the on/off state of the switching element driving the heater.
However, a current supplied from the commercial power supply to the fixing apparatus needs to be controlled to a rated current (protection circuit) of the fixing apparatus and a current value equal to or less than the upper limit defined by Underwriters Laboratories Inc. (UL) or Electrical Appliance and Material Safety Law. Therefore, there is an apparatus for detecting a current flowing in the fixing apparatus and controlling the power supplied to the fixing apparatus so as not to exceed the upper limit value of the current that may be caused to flow. Hence, in recent years, printers increasingly need to be provided with a circuit for detecting the current flowing in the fixing apparatus.
Japanese Patent Application Laid-Open No. 2004-226557 and Japanese Patent Application Laid-Open No. 2004-309518 propose methods of detecting an effective current value on a half period basis by inputting a wave form obtained by voltage-transform by a current detection transformer into a current detection circuit via a resistor. In general, a secondary-side voltage wave form obtained by voltage-transform by the current detection transformer generates distortion due to the inherent characteristics of the element. When a distorted voltage wave form is input to the current detection circuit, the effective value of the wave form changes due to the distortion, which lowers detection precision of the current detection circuit. Note that, the amount of distortion generated in the current detection transformer varies depending upon the amplitude, the phase angle, and the frequency of a primary-side input wave form. In particular, if there is steep fluctuation in the load, the amount of distortion generated in the current detection transformer increases.
The power supplied to the heater is steadily increasing owing to the recent enhancement of printing speed. Further, the regulation of flicker, the regulation of a harmonic current, and other such regulation, which are becoming more stringent, are harder to comply with only by the conventional heater power control using one of the phase control and the wave number control. In contrast, the control method combining the phase control and the wave number control is effective.
However, particularly in the above-mentioned method combining the phase control and the wave number control, the fluctuation in load is larger than in the conventional phase control because the phase control and the wave number control are changed over in one control period, and hence it is difficult to detect a current with accuracy.